It is known to produce aldehydes and alcohols, which contain one carbon atom more than the starting olefin, by reacting olefins with carbon monoxide and hydrogen (hydroformylation). The reaction is catalyzed by hydrido-metal carbonyls, preferably those of the metals of Group VIII of the Periodic Table (IUPAC Version). Apart from cobalt, which is widely used industrially as a catalyst metal, rhodium has recently been gaining increasing importance. In contrast to cobalt, rhodium allows the reaction to be carried out at a low pressure; furthermore, preferentially straight-chain n-aldehydes are formed, with an only minor fraction of iso-aldehydes. Finally, the hydrogenation of the olefins to give saturated hydrocarbons in the presence of rhodium catalysts is also markedly less extensive than with cobalt catalysts.
In the processes accepted in industry, the rhodium catalyst is employed in the form of modified hydrido-rhodium carbonyls which contain additional ligands, especially tertiary organic phosphines or phosphites. In most cases, there is an excess of the ligands, so that the catalyst system is composed of the complex compound and free ligand. The use of the rhodium catalysts described allows the hydroformylation reaction to be carried out at pressures below 30 MPa.
In this process, however, it is difficult to separate the reaction products and to recover the catalysts which are homogeneously dissolved in the reaction product. In general, the reaction product is distilled for this purpose out of the reaction mixture. In practice, however, because of the thermal sensitivity of the aldehydes and alcohols formed, this approach is feasible only in the hydroformylation of the lower olefins, i.e. olefins having up to about 8 carbon atoms in the molecule. In addition, it has been found that thermal stress on the distillation material also leads to considerable catalyst losses due to decomposition of the rhodium complex compounds.
The drawbacks described are avoided by the use of catalyst systems which are soluble in water. Such catalysts have been described, for example, in German Patent 26 27 354. The solubility of the rhodium complex compounds is achieved by the use of sulfonated triaryl-phosphines as a complexing constituent. In this process variant, the catalyst is separated from the reaction product after completion of the reaction, simply by separating the aqueous and organic phase; i.e. without distillation and hence without additional thermal process steps. A further feature of this procedure is that the n-aldehydes are formed with high selectivity from terminal olefins, with only very minor quantities of isoaldehyes. Sulfonated triarylphosphines and, in addition, carboxylated triarylphosphines are preferably used as complexing constituents of water-soluble rhodium complex compounds.
The known two-phase processes have proven to be highly suitable on an industrial scale. Nevertheless, efforts are being made to perfect the process even further. Thus, the prior art attempted to increase the activity of the catalysts by modification of the complex ligands and to extend their activity to further reduce the specific catalyst requirement--both rhodium and ligand--and hence the production costs. Economic factors are also the reason for working towards a marked reduction in the phosphine/rhodium ratio. A further improvement in the hitherto achieved high selectivity with respect to the formation of non-branched aldehydes is also desired. Several million tons of hydroformylation products are manufactured per year, so that even a small increase in the selectivity has economically significant consequences.
It is the object of the invention to improve the hydroformylation process as outlined above, i.e. to develop catalysts which exceed the activity and selectivity of known catalysts at a lower possible ligand/rhodium ratio.